This invention relates to anodes for electrolytic cells for the production of aluminum, and specifically to a method to reduce anode stud corrosion which will result in a reduction in anode voltage losses, labor required to reset anode studs and stud maintenance costs and an improvement in anode and cell performance.
A commonly utilized electrolytic cell for the manufacture of aluminum is of the classic Hall-Heroult design, utilizing carbon anodes and a substantially flat carbon-lined bottom which functions as part of the cathodic system. The electrolyte used in the production of aluminum by electrolytic reduction of alumina consists primarily of molten cryolite with dissolved alumina, and may contain other material such as fluorspar, aluminum fluoride, and other metal fluoride salts. Molten aluminum resulting from the reduction of alumina is most frequently permitted to accumulate in the bottom of the receptacle forming the electrolytic cell, as a molten metal pad or pool over the carbon-lined bottom, thus acting as a liquid metal cathode. Carbon anodes extending into the receptacle from above, and contacting the molten electrolyte, are adjusted relative to the liquid metal cathode. Clark collector bars, frequently of steel, are often embedded in the carbon-lined cell bottom, completing the connection to the cathodic system. Similarly the commonly utilized carbon anodes are physically and electrically connected to anode studs, most often of steel, which are suitably raised and lowered as necessitated by the oxidization of the carbon anode and the necessary renewal thereof.
The electrolyte contained in the electrolytic cell forms a solid crust where exposed to the cooler atmosphere above the electrolyte, which in turn is covered with a layer of alumina for periodical enrichment of the electrolyte and thermal insulation of the bath in the electrolyte pot. The anodes, consisting of carbon, penetrate the alumina layer and the crust, extending into the electrolyte, for conduction of the electric current which maintains the electrolysis. The crust, and the aluminum oxide deposited thereon, normally do not form a gas-type seal around the circumference of each anode, due to rising gases and motion of the molten electrolyte. In addition, the crust is periodically broken for enrichment of the electrolyte with alumina.
The gases released from the electrolytic process, primarily a mixture of gaseous fluorides, carbon dioxide, CO.sub.2, and carbon monoxide, CO, penetrate the carbon anode through cracks and open porosity within the carbon anode. These gases can react with chemical components within the anode to form a corrosive gas such as CO+S=COS, carbonyl sulfide. The anode gas and/or gaseous products are corrosive to the anode studs supporting the carbon anodes and providing electrical connection thereto. The temperatures within the anode can range from 100.degree. C. or greater at the top of the anode to the temperature of the electrolyte 900.degree. to 1000.degree. C. at anode lower surface. Thus the anode stud, normally an uprotected steel surface, is subjected to highly corrosive gases at temperatures which expedite corrosion and deterioration of such materials.
Although considerable effort has been expended to protect other components of the electrolytic cell, such as the electrodes themselves, we are not aware of any satisfactory method to reduce anode stud corrosion. It is known that corrosion of the anode studs in a vertical stud Soderberg aluminum reduction cell contributes directly to increased stud maintenance cost and power consumption, as well as reduced metal quality and cell performance. The anode studs corrode, forming a scale containing various forms of iron sulfide and iron carbide. It has been shown that one corrosive agent involved is carbonyl sulfide, COS, which forms in a reaction between CO and the sulfur in the anode carbonaceous materials. During stud pulling, i.e., removal of the stud for resetting to a greater distance from the anode face, pieces of the scale remain in the anode, which in time are transferred qualitatively into the metal. It has also been observed that the iron content in the aluminum metal produced was a direct function of the sulfur content of the anode materials in Soderberg cell operation. It has previously been reported that low alloy steels corrode significantly less than ordinary carbon steels when exposed to a sulfur-bearing anode mass, although later results disputed this reported improvement. A steel stud coated with aluminum appears to be protected from attack by a fluoride-free sulfur-bearing anode mass. However, with the introduction of amounts of volatile fluoride, known to be present in actual anode gases, such aluminum coating and stud material were heavily corroded.
Formation of a poor electrically conducting iron sulfide film on an anode stud increases the cell voltage loss in the anode and consequently increases the energy required to produce aluminum. The increased stud to carbon contact resistance produces local non-uniformity in the anode current distribution, which can initiate and/or enhance the formation of anode spikes, which can short-circuit through the metal pad causing severe local heating within the anode. Thus, it is desirable to prevent the formation of this scale or film. It is noted that such short-circuiting can result in the melting of several inches of iron from the stud tip, and the formation of small metal globules of iron in the anode carbon.
Development of a cost-effective, corrosion-resistant, electric-conductive stud coating would clearly help to reduce cell energy requirements, improve metal quality, and permit the use of low-cost, high sulfur-content anode carbonaceous materials. In addition, anode stud maintenance would be reduced, consequently simplifying the stud resetting process, hence further reducing operating costs.